The present invention relates to a thin-walled aluminum alloy composite material to be used for automotive heat exchangers or the like. More particularly, the present invention relates to an aluminum alloy composite material, of three-layer structure, to be made into tubes as refrigerant passages of a heat exchanger formed by brazing. The present invention also relates to an aluminum alloy to be used as a sacrificial material for the said aluminum alloy composite material. Further, the present invention relates to a heat exchanger using the said aluminum alloy composite material.
The conventional heat exchanger made of aluminum alloy, as exemplified by the radiator shown in FIGS. 1(A) and 1(B), is constructed as follows. Tubes (1), for refrigerant to pass through, are joined together by fins (2) arranged between them. Both ends of the tubes (1) are provided with header plates (3). Thus, a core (4) is assembled. After brazing, both header plates (3) are provided with resin tanks (5a) and (5b), respectively, with packings (6) interposed between them. The fins (2) are formed from a sheet (about 0.1 mm thick) of JIS 3003 alloy incorporated with about 1.5 wt % of Zn. For protection from pitting corrosion in contact with refrigerant, the tubes are formed from a brazing sheet (0.2 to 0.3 mm thick) consisting of a core and an inside (refrigerant side) layer cladding the core. The core is w formed from JIS 3003 alloy incorporated with Si, Cu, or the like. The inside layer is formed from JIS 7072 alloy (or JIS 7072 alloy incorporated with Mg), as a sacrificial anode material. The header plate (3) is also formed from a brazing sheet (1.0 to 1.3 mm thick) of the same material used for the tubes (1).
The brazing sheet undergoes brazing in a hot atmosphere at about 580 to 610xc2x0 C. Brazing causes Zn in the sacrificial anode material to diffuse into the core, as shown in FIGS. 2(A) and 2(B). The Zn-diffused layer suffers corrosion preferentially, so that pitting corrosion that occurs on the surface in contact with refrigerant does not grow deep. Thus, shallow and wide pitting corrosion occurs, and the brazing sheet exhibits good resistance to pitting corrosion for a long period of time. Shallow and wide pitting corrosion (i.e., uniform corrosion) is characteristic of the sacrificial anode alloy of Alxe2x80x94Zn-series (with Zn-amount usually being 3 wt % or less), Alxe2x80x94Znxe2x80x94Mg-series, or Alxe2x80x94Mgxe2x80x94In-series. Owing to the potential difference between the core material and the sacrificial material, the sacrificial material continues to suffer corrosion preferentially even after the core has been exposed. It is presumed that the core is protected from corrosion in this way.
Recently, the flow rate of liquid in tubes has been increased, because of thinner plates for weight reduction and the requirement of high-performance of heat exchangers. Further, according to cooling water (coolant) for use, tubes are designed for strongly alkaline liquid. According to running conditions of automobiles, a large problem: That conventional sacrificial materials do not produce the effect of corrosion protection enough, and tubes are subject to erosion as well as corrosion, when the function of coolant becomes deteriorated for a certain reason, arises.
A conventional corrosion-resistant brazing sheet for heat exchangers is disclosed in JP-A No. Hei 9-87788 (xe2x80x9cJP-Axe2x80x9d means unexamined published Japanese patent application). The document discloses a sacrificial anode material of aluminum alloy containing 6 to 12 wt % of Zn and 0.5 to 3 wt % of Mg. This content of Zn overlaps the Zn content in the sacrificial corrosion-protective aluminum alloy of the present invention. However, these aluminum alloys entirely differ in additive elements except for Zn. The aforesaid document mentions the protection against corrosion caused by strongly alkaline coolant, but it mentions nothing about the protection against corrosion under a severer situation caused by fast-flowing liquid. The composition specified for the sacrificial material in the aforesaid document is not intended to propose a sacrificial material not only to solve the problems of corrosion but also that associated with erosion.
Other sacrificial anode materials are disclosed in JP-A Nos. Hei 9-176768 and Hei 10-72634. They are made of aluminum composite materials superior in alkaline corrosion resistance, which contain 0.1 to 3.0 wt % of Ni, 0.5 to 3.0 wt % of Fe, and also 3.0 wt % or less, or 4.0 wt % or less of Zn. These materials contain less Zn than the material of the present invention. In addition, no mention is made about their corrosion protection against fast-flowing liquid. The effects of their additive elements, especially the synergistic effects of Fe, Ni, and Zn, are different from the effects of the additive elements of the present invention.
It is an object of the present invention to provide an aluminum alloy composite material to be used for heat exchangers whose tubes are less subject to erosion when their inside is exposed to strongly alkaline refrigerant and even when the liquid flows fast.
It is another object of the present invention to provide a corrosion-protective aluminum alloy to be used as the sacrificial material for the said aluminum alloy composite material.
It is further another object of the present. invention to provide a heat exchanger using the said aluminum alloy composite material.
Other and further objects, features, and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.